/Engine/Source/Runtime/Engine/Public/Slate/SceneViewport.h
Updates the viewport RHI with a new size and fullscreen flag
virtual void UpdateViewportRHI ( bool bDestroyed, uint32 NewSizeX, uint32 NewSizeY, [EWindowMode::Type](API\Runtime\ApplicationCore\GenericPlatform\EWindowMode__Type) NewWindowMode, EPixelFormat PreferredPixelFormat )